Herner et al., U.S. patent application Ser. No. 10/955,549 filed Sep. 29, 2004 (which corresponds to US Published Application 2005/0052915 A1), hereby incorporated by reference, describes a three dimensional memory array in which the data state of a memory cell is stored in the resistivity state of the polycrystalline semiconductor material of a pillar shaped semiconductor junction diode. A subtractive method is used to fabricate such pillar diode devices. This method includes depositing one or more silicon, germanium or other semiconductor material layers. The deposited semiconductor layer or layers are then etched to obtain semiconductor pillars. A SiO2 layer can be used as a hard mask for the pillar etching and removed afterwards. Next, SiO2 or other gap fill dielectric material is deposited in between and on top of the pillars. A chemical mechanical polishing (CMP) or etchback step is then conducted to planarize the gap fill dielectric with the upper surface of the pillars.
For additional description of the subtractive pillar fabrication process, see Herner et al., U.S. patent application Ser. No. 11/015,824, “Nonvolatile Memory Cell Comprising a Reduced Height Vertical Diode,” filed Dec. 17, 2004 and U.S. patent application Ser. No. 11/819,078 filed Jul. 25, 2007. However, in the subtractive method, the height of the semiconductor pillar may be limited by thin and soft photoresist used as the etching mask. The photoresist mask material etches at a slower rate than the semiconductor material, but etches nonetheless, and some mask material must remain when the semiconductor etch is completed. The oxide gap filling step after pillar etch presents a processing challenge when the aspect ratios of the openings between the pillars increases and/or the CMP process or etchback of the gap fill layer removes a significant thickness of the deposited semiconductor material. Finally, as the pillar size the decreases, the mechanical strength of the pillar formed by the substrative method also decreases.